Impact of Mixing an Aqueous Capsule Suspension Formulation of Pendimethalin and Glyphosate on Growth and Yield of Second-Generation Glyphosate-Resistant Cotton

Past research has shown that a postemergence (POST) mixture of a residual herbicide, such as S-metolachlor, with glyphosate can result in improved weed control and maximize cotton yield. Published research is limited on the tolerance of a POST mixture of an aqueous capsule suspension (ACS) formulation of pendimethalin with glyphosate applied at different growth stages to cotton. Research of this issue is important as producers continue to search for cost savings in cotton production, and the inclusion of residual herbicides such as pendimethalin in the planting regimen is needed to abate the expansion of glyphosate-resistant (GR) weeds. This research was conducted in 2006 and 2007 to evaluate the effects of POST combinations of glyphosate with an ACS formulation of pendimethalin on second-generation GR cotton growth, development, and yield. Treatments evaluated in the study included a factorial arrangement of herbicides [glyphosate (®Roundup Weathermax) at 1540 g ha-1 alone or in combination with ACS pendimethalin (®Prowl H2O) at 1064 or 2128 g ha-1 or S-metolachlor (Dual Magnum) at 1064 or 2128 g ha-1] and cotton growth stage (4- to 5- or 6- to 8-leaf). Ammonium sulfate at 2524 g ha-1 was included with each treatment. Results indicated that visual crop response in the form of veinal yellowing and slight leaf malformation can be observed following a POST tank-mixture of pendimethalin with glyphosate. Response is limited to contacted leaves. Visual crop response following mixture of glyphosate with pendimethalin was minimal 21 d after both application timings and was not manifested in reductions in plant height or seedcotton yield compared to glyphosate applied alone.